Hiwatt DR103 build - Voltages too low

[Lifto]

Hi all,

I just completed my Hiwatt DR103 copy. The amp fires up, fuses won't blow, the indicator lights up and the tube heaters light up.
There is however no sound. To find out what is going inside the amp I measured the voltages on the tubes and found that the voltages in the preamp are way to low. Below I will give an overview of the voltages at the different tubes. What should be my next step in debugging this build? First half of V2 is unused in this circuit.

V1:
1: 37.7 2:x 3: 0.123 6: 31.4 7:x 8:0.17

V2:
1: X 2: X 3: X 6: 12.2 7: X 8: X

V4 (PI 12at7):
1: 46.7 2: 12.1 3: 12.6 6: 58.8 7: 5.9 8: 12.6

V3:
1 52.4 2: 8.6 3: 12.2 6: 30.3 7:X 8: 0.42

Power tubes (EHX 6ca7)
V1 3: 482 4: 77.3 5: -23.5 6: 76
V2 3: 482 4: 76.5 5: -23.3 6: 76.5
V3 3: 485 4: 77.1 5: -32.2 6: 76
V4 3: 485 4: 76.6 5: -32.5 6: 76

On the bottom of M. Huss DIY page I found some voltages:
MHuss Hiwatt Amplifier Pages - Hiwatts DIY

I used this layout:
http://hiwatt.org/Layouts/DR103hiwattlayout.pdf

Schematics:
preamp:http://hiwatt.org/Schematics/DR_Pre4Input_v1a.pdff
output stage: http://hiwatt.org/Schematics/DR_100wOutput.pdf
power supply: http://hiwatt.org/Schematics/DR_100wPS.pdf

I hope some of the folks here can give me a hint where to look first. As always many thanks already. Without this online community I would have a lot of semi finished amps!

[Jazz P Bass]

Yes, your preamp voltages are low, as is the output tube screen voltage.

You should be able to suss this out with the power off, caps drained.

Use the power supply link that you provided & start checking everything.

Standby switch off: Bridge rectifier side of switch/ 440K ohms to ground.
HT1 to HT2: 570 ohms
HT2 to HT3: 1470 ohms
HT3 to ground (Standby off) 440K ohms.

And double (no triple) check that the power supply capacitors are in the correct orientation.

[Lifto]

Thanks! I just did some measuring.
Standby tot bridge rectifier measured good (+-438k)
HT1 tot HT2 measured 570r
However I couldn't get a measurement from HT2 to HT3. Upon inspection I found out that the 1k 5w resistor (between the 2x 50uf caps) was showing signs of overheating. I also can't tell what the 3rd band on the resistor is. Could be brown black or violet. But not red.
I took the resistor out and when measuring it, the resistance fluctuates between 500k-600k! I put in a 2 watt resistor since this is specified in the layout: http://hiwatt.org/Layouts/PowerAmpLayout100.pdf
However, the schematic calls for a 1k 5watt resistor.
http://hiwatt.org/Schematics/DR_100wPS.pdf
Guess I'll put in a 5w version in it and will report back!

[Jazz P Bass]

You need to find Why the resistor got hot.

My first suspect would be cap polarity.

The resistor reading a high value does not explain the low screen voltage (HT2), unless something is pulling hard on H3.

[g1]

The resistor reading a high value does not explain the low screen voltage (HT2), unless something is pulling hard on H3.

For sure. There seem to be several problems. A 2W resistor should not be overheating, HT3 only running the preamp tubes.
HT2 has a problem as it is low.
Bias supply also seems to have a problem as schematic shows -38V but you have -32V (low) and -22 (real low).

[Lifto]

Ah yes, you acknowledge what I already thought. I need to work more carefully and start double/triple checking all the wiring! Upon first firing it up I did mistake the polarity of the 100uf/63v cap. I corrected for the polarity but it might've already done some damage. I ordered some new parts (new resistors, cap etc.) which will arrive tuesday. This leaves me some time to inspect my wiring. Thanks for putting me on the right track!
I already found 1 wiring mistake in the bias section. I did not wire the 47k resistor to ground but instead connected it to the 10n nfb cap (which should have been tied to ground aswell). In the layout it isn't quite clear how it is wired but looking in the schematic helped.
I'll keep you updated.

[Jazz P Bass]

Layout diagrams are fine.
They have there use.

The schematic is the bible.

[Lifto]

Hi all,

Reporting back like I said. I managed to get the amp working. I like the sounds there is only quite a bit of hum present. At first I thought this would probably have to do with lead dress and might be caused by heater wiring. Instead of thinking and measuring before acting I started changing stuff, as you can guess with no results.
Today I was having a good day and managed to think about probable causes. Since the hum is always present (channel volumes/master volume do not really change the amount of hum) it probably comes from the power amp section. I measured the voltages on the output tubes and found a large difference on pin 5 for the pairs. The 2 tubes on the left have -39 on pin 5 (where the schematic calls for -38) but the 2 tubes on the right measures -51! I guess the hum might be caused the tubes being severly unmatched bias-wise. First thought would be that this is a wiring mistake (wouldn't be my first). However when measuring from the bias tap to each of the pin 5's all tubes measures 122k (which it should be) (see: http://hiwatt.org/Schematics/DR_100wOutput.pdf)

Does anyone have a clue where I should start looking next?

I attached some pictured of my build. I really enjoyed this build. Although there is a a lot to improve I already learned a great deal building this amp. The amp used to be neater but my brainless troubleshooting caused some wires to be relocated.

Thanks for all the help already!

[loudthud]

The first test to do would be to swap the tubes from side to side to see of the bad pin 5 voltage follows the tube or stays with the socket.

[Lifto]

Reporting back here!

I swapped the tubes around and tested the voltages without power tubes. The bias difference stays with the two outer sockets. In the meanwhile I've done some more testing and honestly don't know where to look. The bias comes in from the transformer and goes trough a diode followed by a 1k resistor. After the 1k resistor the current splits and goes trough a 100k resistor for the 2 left tubes and a 100k resistor for the 2 right tubes. I measured the current before the 100k resistors. At that point the current is the same. The voltages difference occurs after the 100k resistor. The 100k resistor on the right measures approx 75k when it is installed. After I found this out I replaced the right 100k resistor to no avail. After I took the resistor out it measured 100k. I also replaced the 100k resistor to an empty spot on the tagboard to rule out it had anything to do the 100uf/63v capacitor it shared a turret with on the tagboard. What can cause the resistor value to lower once it is installed?

Here is the layout: http://hiwatt.org/Layouts/PowerAmpLayout100.pdf
Schematic Output: http://hiwatt.org/Schematics/DR_100wOutput.pdf
Schematic PS: http://hiwatt.org/Schematics/DR_100wPS.pdf

Maybe I will just rebuild the small tagboard strip.

[mhuss]

Nice site you're referring to.

Lift one end of the suspect resistor and measure it again. I suspect you have something else going on in the circuit that's pulling the apparent value down. With -5 on the tube grid, it's almost certainly red-plating .. don't keep it on too long!

The left two tubes connect to one 100k and the right two connect to the other. Any difference should be left/right, not inner/outer.

--mark

[g1]

The left two tubes connect to one 100k and the right two connect to the other. Any difference should be left/right, not inner/outer

I think that inner outer may be a typo. Back in post #8 the imbalance was between the 2 left and the 2 right.
I would suspect the coupling cap feeding the 2 sockets with the lower grid voltage (-39V) may be leaky. Being a new build, if the caps are new there shouldn't be any leakage, but it's worth a try replacing it.

[Lifto]

Hi all,

The nice weather and work has kept me from replying (and my soldering iron) up until now.

Thanks for checking in Mark. Without your website I would've never started this built. Your website is a valuable source for schematics and layouts. (I have actually mailed you once when a noticed something missing from the layout. Now I mention it I actually now another connection missing in one of the schematics. If you would like I can mail you about it).

The outer/inner tubes is indeed (as g-one noticed) a typo. I meant the 2 tubes on the left and the 2 on the right.

It is indeed a new build but I check the capacitors.

Cheers!
Youri